Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly ("MEA") which comprises an ion exchange membrane or solid polymer electrolyte disposed between two electrodes formed of porous, electrically conductive sheet material, typically carbon fiber paper. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes to an external load.
At the anode, the fuel permeates the porous electrode material and reacts at the catalyst layer to form cations, which migrate through the membrane to the cathode. At the cathode, the oxygen-containing gas supply reacts at the catalyst layer to form anions. The anions formed at the cathode react with the cations to form a reaction product.
In electrochemical fuel cells employing hydrogen as the fuel and oxygen-containing air (or substantially pure oxygen) as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of hydrogen ions from the anode to the cathode. In addition to conducting hydrogen ions, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode, oxygen reacts at the catalyst layer to form anions. The anions formed at the cathode react with the hydrogen ions that have crossed the membrane to form liquid water as the reaction product. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations:
Anode reaction: H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- PA1 Cathode reaction: 1/2O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O PA1 (a) a substantially fluid impermeable housing; PA1 (b) means for introducing at least one inlet fluid stream to each of the fuel cell stacks; PA1 (c) means for exhausting at least one outlet fluid stream from each of the fuel cell stacks.
In typical fuel cells, the MEA is disposed between two electrically conductive plates, each of which has at least one flow passage engraved or milled therein. These fluid flow field plates are typically formed of graphite. The flow passages direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. In a single cell arrangement, fluid flow field plates are provided on each of the anode and cathode sides. The plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant to the respective anode and cathode surfaces, and provide channels for the removal of water formed during operation of the cell.
Two or more fuel cells can be connected together, generally in series but sometimes in parallel, to increase the overall power output of the assembly. In series arrangements, one side of a given plate serves as an anode plate for one cell and the other side of the plate can serve as the cathode plate for the adjacent cell. Such a series connected multiple fuel cell arrangement is referred to as a fuel cell stack, and is usually held together in its assembled state by tie rods and end plates. The stack typically includes manifolds and inlet ports for directing the fuel (substantially pure hydrogen, methanol reformate or natural gas reformate) and the oxidant (substantially pure oxygen or oxygen-containing air) to the anode and cathode flow field channels. The stack also usually includes a manifold and inlet port for directing the coolant fluid, typically water, to interior channels within the stack to absorb heat generated by the exothermic reaction of hydrogen and oxygen within the fuel cells. The stack also generally includes exhaust manifolds and outlet ports for expelling the unreacted fuel and oxidant gases, each carrying entrained water, as well as an exhaust manifold and outlet port for the coolant water exiting the stack.
Two or more fuel cell stacks can be electrically connected, generally in series but also in parallel, to increase the overall power output of the system. Such a series connected multiple fuel cell stack arrangement is referred to as a fuel cell stack array.
In conventional fuel cell stack arrays, the inlet fuel (substantially pure hydrogen, methanol reformate or natural gas reformate), oxidant (substantially pure oxygen or oxygen-containing air), and coolant streams are generally provided to the individual fuel cell stacks by separate, external inlet conduits, each of which serves a single stack. Similarly, the outlet fuel, oxidant and coolant streams are generally exhausted from the stacks by separate, external outlet conduits, each serving a single stack. Conventional arrays therefore typically have a complex network of reactant and coolant feed and exhaust conduits associated with them. Such a complex network of conduits makes servicing the individual stacks difficult in that each conduit must be separately identified, disconnected from the corresponding inlet or outlet port, and then reconnected upon the completion of servicing. A complex network of separate, external feed and exhaust conduits must also be afforded a significant amount of volume, not only because of the overall space occupied by conduits, but also because of the space required to access and manipulate the conduits and their corresponding inlet and outlet ports during servicing.
Accordingly, it is an object of the invention to provide a fluid manifold assembly for a fuel cell stack array which reduces the number and complexity of components for delivering the inlet reactant and coolant streams to the stacks and for exhausting the outlet reactant and coolant streams from the stacks.
It is also an object of the present invention to provide a fluid manifold assembly for a fuel cell stack array which improves the volumetric efficiency of the array.